MSM type photodetectors on a GaAs substrate have operating characteristics (dark current, speed, sensitivity) that are entirely satisfactory and they are completely compatible technologically with MESFET type field-effect transistors. Components of this type have been developed in particular in the form of integrated photoreceivers and more particularly in the form of monolithic strips of photoreceivers. In this respect, reference may advantageously be made to:
[1] J. Choi et al., "High-performance high-yield, uniform 32-channel optical receiver array", OFC'96 Technical Digest, pp. 309-310. PA1 [2] H. T. Griem et al., "Long-wavelength (1.0-1.6 .mu.m) In.sub.0.52 Al.sub.0.48 As / In.sub.0.53 (Ga.sub.x Al.sub.1-x).sub.0.47 As / In.sub.0.53 Ga.sub.0.47 As metal-semiconductor-metal photodetector", Appl. Phys. Lett. 56 (11), Mar. 12, 1990, pp. 1067-1068; and PA1 [3] M. C. Hargis et al., "Temporal and spectral characteristics of back-illuminated InGaAs metal-semiconductor-metal photodetectors", IEEE Photon. Technol. Lett. Vol. 8 (1996), No. 1, pp. 110-112. PA1 [4] C. X. Shi et al., "High-performance undoped InP/n-In.sub.0.53 Ga.sub.0.47 As MSM photodetectors grown by LP-MOVPE", IEEE Trans. Electron. Dev. Vol. 39, No. 5, May 5, 1992, pp. 1028-1030; and PA1 [5] R. H. Yuang et al., "High-performance large-area InGaAs MSM photodetectors with a pseudomorphic InGaP cap layer", IEEE Photon. Technol. Lett. Vol. 7, No. 8 (1995), pp. 914-916.
Since GaAs is not photosensitive at 1.3 .mu.m and at 1.55 .mu.m, various laboratories have investigated MSM photodetectors having a GaInAs absorbent layer 2. As shown in FIGS. 1 and 2, such photodetectors are generally constituted by a stack of a plurality of layers grown epitaxiallly on a substrate 3 of indium phosphide (InP). Above a possible buffer layer 4 intended to keep the active zone of the component apart from surface defects of the substrate, there can be found an active layer 2 that is photosensitive and is made of GaInAs, followed by a transition layer 5 of graded composition made of AlGaInAs, and finally a barrier layer 6 of AlInAs. The graded composition layer may be obtained in various ways, by continuously varying composition or by juxtaposing a plurality of fine layers having given compositions, for example.
Two very fine electrode combs 1 (of width &lt;1 .mu.m) that are interdigitated with small spacing (about 1 .mu.m to 3 .mu.m) are then deposited on the barrier layer 6. These electrodes 1 co-operate with the semiconductor material that constitutes Schottky type junctions, collecting the carriers created by photons being absorbed in the semi-conductor.
Embodiments of this kind are described, in particular, in the following:
Apart from the substrate which is generally doped with iron (Fe) in order to make it semi-insulating, the set of layers 3, 4, 5, and 6 is generally not doped. It should be observed that other materials have been mentioned in the literature for making the barrier layer 6, such as InP that is not doped, that is of p type, or that is semi-insulating (Fe doped). In this respect, reference may be made to the following:
In the structure of FIGS. 1 and 2, AlGaInAs transition layer 5 serves to eliminate carriers being trapped at the AlInAs/GaInAs heterojunctions; the graded nature of the composition makes it possible to provide a progressive transition from the conduction and valence bands of GaInAs to those of AlInAs. Nevertheless, it is necessary to apply a large voltage to ensure that the barriers which oppose electrons and holes collecting at the contacts disappear completely.
FIG. 3 shows the electric field lines E of a component of the type shown in FIGS. 1 and 2.
Under normal conditions, both metal/semiconductor junctions are biased, one of them forwards and the other one reversed.
Applying a high bias voltage as is required in order to obtain a large passband is naturally harmful with respect to obtaining low dark current, and that degrades the overall performance of the detector. In addition, reliability is certainly degraded in that device where the highest electric fields are on the surface and apply to all of the surface of the photodetector. Finally, the use of voltages higher than those required for proper operation of the associated electronic circuits is always penalizing, from the point of view of the electricity consumed by the receiver module.
In the special case of AlInAs/GaInAs photodiodes having n-type residual doping for the layers 2 of GaInAs, it is the configuration of the bands adjacent to the positive electrode 1 that plays a major role. This electrode 1 collects the electrons and it is therefore the discontinuity between the conduction bands (0.55 eV) that determines the voltage that needs to be applied. For example, for a transition layer having a thickness of 200 nm, it is necessary to have an electric field of 0.55/200 V/nm, i.e. 27.5 kV/cm to eliminate the barrier opposing electron collection. Given the residual doping of the absorbent layer 2 and the distance between the electrodes 1 which is typically 1 .mu.m to 2 .mu.m, it is difficult to achieve such a field at low voltage (at a bias of 5 V, the field provided by the external voltage is still negligible). A smaller field would suffice if the transition layer were thicker, however that would be to the detriment of the overall speed of the device since the photosensitive zone would then be further away from the electrodes. In practice, and for an absorbent layer 2 having a thickness of 1 .mu.m to 2 .mu.m (necessary for good absorption at 1.3 .mu.m and at 1.5 .mu.m) and for electrodes that are spaced apart by 2 .mu.m, it is necessary to apply about 15 V in order to obtain satisfactory bandpass characteristics. At lower voltages, carriers become trapped, thereby restricting the passband and giving rise to very long smearing in the impulse response.